Summary It is well established that changes in cellular phenotypes that occur during development are highly dependent on the patterning and distribution of epigenetic marks in the genome. Indeed, the major focus of mechanistic epigenetic studies has centered around the interplay between histone or DNA modifying proteins and transcription factors. These studies have been immensely fruitful and yielded many fundamental insights into gene regulation in health and disease. Yet the enormous length of the genome and the requirement for large sets of genes to be regulated and transcribed in a coordinated manner poses significant energetic and physical challenges to cells. It is thus not surprising that an additional critical level of control of the epigenome is conferred through the three dimensional structure of chromosomes and their organization within the nucleus. Though it is known that 3D genome organization plays a crucial role in gene regulation and cancer, the underlying mechanisms connecting these are poorly understood. CTCF is central to these, as it governs genome organization and is implicated in cancer. In fact mutations in CTCF are detected in numerous cancers, however the extent to which they perturb 3D chromosomal architecture and contribute to the malignant phenotype is unknown. We hypothesize that each CTCF mutation will alter cellular function in a different manner depending on whether it is associated with (i) total loss of CTCF binding, (ii) a change in binding affinity, (iii) an alteration in binding motif preference or (iv) orientation of binding. Furthermore, we propose that mutations, which occur frequently in cancers that have no apparent effect on binding or binding affinity will have important functions in disrupting dimerization of CTCF molecules or binding of important cofactors such as cohesin. To test these models we aim to use three innovative approaches that examine the impact of mutations on (i) binding affinity and target sequence specificity, (ii) chromosome structure and gene regulation, and (iii) in vivo phenotype and tumor initiation/progression. We will start by characterizing the impact of cancer associated CTCF mutations on their binding affinity and binding motif. Next we will analyze their functional effect on cell survival, gene expression and chromosome organization. Finally, mouse models will address the key question of whether cancer associated CTCF mutations alone can predispose to malignant transformation or whether cooperating mutations are required. We propose that gaining a mechanistic predictive understanding of the impact of CTCF cancer associated mutants is essential to understand cancer genomes. This may enable a range of novel therapeutic approaches to counteract the malignancy effects of mutant architectural proteins. 1